The present invention is an archery bow which represents an improvement over my U.S. Pat. Nos. 4,903,677 and 5,054,463. The object of those patents was to provide a compound type bow of a more compact size, as compared to conventional compound type bows, while offering all of the features of a full size bow with respect to performance, range of draw length, accuracy, and other parameters.
I have found, however, that while my prior patents do indeed provide ways of reducing the overall size of a compound type bow dramatically without sacrificing the ability to provide the longest draw length required, certain aspects of the design act to inhibit performance while other aspects enhance performance, but not enough to compensate for the performance-degrading aspects.
The present invention provides an improved compound bow incorporating those features of the previous design that successfully accomplish the goal of providing for ultra-compact size while improving the bow by eliminating those characteristics of the previous design that were detrimental to performance.
In order for a compound bow to be successful, it must be capable of producing an acceptable level of performance in terms of both arrow velocity and accuracy. Acceptable performance with respect to arrow velocity is not a subjective issue: arrow velocity must meet standards established by the Archery Manufacturers Organization (AMO) to be successful in the marketplace. The AMO standard for determining arrow velocity specifies using a 60 lb. peak draw weight, a 30 in. draw, and a 540 grain arrow. Recent bows tested under this standard produce arrow velocities in the range of 200 to 250 feet per second. My prior designs could not meet this velocity standard.
Accuracy, on the other hand, can be considered subjective, because the accuracy obtainable with any given bow is the product not of the bow alone, but rather the bow/arrow/archer combination. However, some characteristics of a bow design tend to enhance accuracy, and some may tend to detract from it.
The present invention goes well beyond the correction of deficiencies inherent in my previous design.
In my prior U.S. Pat. No. 4,903,677, flat, wound, power spring type components were used to store energy in the bow. I have found that while this configuration can fulfil its function of propelling an arrow, satisfactory arrow velocity is not attainable because the flat, wound spring produces high levels of hysteresis resulting from of the friction between neighboring coils in operation. The hysteresis exceeds that which would allow an acceptable level of energy to be transmitted to the arrow. In order to benefit from the energy curve produced by a power spring in terms of its relationship to the force/draw characteristics required, the unsprung mass of the power spring must be so great that, as a contributing factor to the overall mass weight of the bow, it is impractical as a means of storing energy.
In one embodiment of U.S. Pat. No. 5,054,063, flexible limbs were used in conjunction with power springs to store energy. This configuration suffers the same deficiency described in the previous paragraph.
Another embodiment in U.S. Pat. No. 5,054,063 eliminated the power springs, and relied completely on flexible limbs to store energy. That configuration eliminated any problems associated with the use of power springs and would appear to provide a design that would more closely simulate the efficiency characteristic of conventional compound bow design. However, although performance was improved, it was still short of acceptable levels because of inherent characteristics that tended to inhibit dynamic efficiency. Those same characteristics are not only present in my previous design, but are actually substantially pronounced as a result of the extremely short axle-to-axle (bowtip-to-bowtip) distance.
Modern compound type archery bows are designed, generally, in one of two configurations. The older configuration, commonly referred to as the two-cam bow, has a cam component located at each limb tip. In a two-cam bow, each cam has two cam lobes of different size and profile. The larger of the two lobes has a length of bowstring entrained around a portion of its perimeter; this portion is extracted during the draw, causing the cam to rotate. The second, and relatively smaller, lobe of the cam anchors a buss cable whose other end is anchored to the limb tip. Upon drawing the bowstring, the buss cables are wound onto the smaller lobes thereby drawing the opposing limb tips closer together. The deflection caused in the limbs represents stored potential energy which propels the arrow upon release of the bowstring.
The newer variety of compound bow is called the one-cam bow. It also deflects the two limbs to store energy. However, the one-cam bow does so with a different arrangement of cables and bowstring. The one-cam bow has only one cam located at the tip of the lower limb, with a simple idler pulley replacing the cam at the tip of the upper limb. The cam of the one-cam bow has three, not two, lobes. One of the three lobes acts much like the lesser lobe of the two-cam bow in that, upon rotation of the cam, a buss cable that extends between that lobe and the upper limb tip, is wound onto the lobe, deflecting the limb in much the same manner as the two-cam arrangement. The remaining two lobes of the one-cam bow cam are of different size and configuration. The larger of the two pays out bowstring to the archer draw much like in the two-cam arrangement. The lesser of the two remaining lobes, however, also pays out bowstring, but to an idler at the upper limb tip where the bowstring is entrained around the idler and represents the feed of the bowstring to the draw. Because one lobe that feeds bowstring is considerably larger in profile than that of the second feeder lobe, the amount of bowstring extending from the cam to the idler is essentially shortened during the rotation of the cam and thus contributes to the deflection of the limbs and the subsequent storing of energy.
The design of the configuration of each cam lobe, whether that of a one or two cam arrangement, defines the amount of energy stored in terms of the incremental measurement of force required to draw the bowstring from its initial position to the position at full draw. Configurations of cams vary and produce different draw/force characteristics representing various levels of stored energy. The criteria used in the design of cams for compound bows is commonly known within the art and, while a certain amount of experimentation may be required, a satisfactory cam design for either a one or two cam bow is relatively easy to obtain.
Great advancements have been made in the design of limbs, riser handles, bowstrings and buss cables, bearings, and other components. However, it is generally accepted that the amount of stored energy that is determined by the cam and subsequently transferred to the arrow is the principal factor in arrow velocity, that is, the more energy delivered to the arrow, the faster it will be propelled. Of course, the unfortunate fact remains that the more energy that is available to the arrow means that the more energy is required to draw the bow. Arrow velocity may vary somewhat from one bow to another, each exhibiting the same level of stored energy and same peak draw weight. This is due to the differences in other factors such as overall bow geometry, levels of hysteresis, and the manner in which the energy is defined in the draw. Nevertheless, it has been accepted that a bow capable of producing a given arrow velocity must store a given amount of energy.
Several factors that affect arrow velocity as well as accuracy and overall performance in a compound type bow have, however, been overlooked or ignored altogether as a result of certain limitations inherent in the conventional design of compound bows.
One such factor, and a surprisingly substantial factor affecting arrow velocity, is that of the relationship of the rotational speed (r.p.m.) of revolving components, to the reaction time of the limbs. This effect may be described, to some extent, in terms of the time required for the limbs to return from their fully deflected position at full draw back to their starting position and the speed at which the cam rotates and how this relates to the speed of travel of the bowstring. The reaction of the limbs returning to brace is essentially the driving mechanism to produce rotation of the cam or cams during launch. The cams' rotational speed defines the speed of bowstring travel as it is retracted onto the cam lobes. This bowstring travel speed is influenced by the ratio of the varying radius of the cam lobe being driven by the limb to that of the lobe that is drawing up bowstring.
In order to achieve a force/draw relationship by virtue of cam design that represents an acceptable level of stored energy to produce an acceptable range of arrow velocity, it is necessary that the relationship of one cam lobe to another be such that the potential energy produced by the deflection of the limbs is defined in terms of the force of draw in a structured manner. In order to achieve such structured characteristics, it is necessary that, from the braced position at the commencement of the draw, the length of radius of the lobe of the cam that directly produces limb deflection is substantially longer than the length of the radius of the lobe from which bowstring is extracted. Throughout the draw, the relationship of the length of one cam lobe radius to the other constantly changes creating a graduated progression of ratio relationships that progressively alter the moments of torque on the cam throughout the rotation of the cam. At full draw, in order to achieve the necessary compounding characteristics of the draw, the length of the radius of the lobe from which bowstring is extracted must now be substantially longer than the length of the radius of the lobe directly deflecting the limb. The basic principles of cam design for compound bows have their origin in the U.S. Pat. No. 3,486,495 issued to H. W. Allen, and are well known. However, while these principles successfully accomplish the goal of defining and controlling the distribution of stored energy throughout the draw, they ignore the effect of the ratio of one cam lobe to the other in terms of the effect on the speed of bowstring travel.
To examine this effect further, we must examine the sequence of events from release of the bowstring to the point at which the arrow leaves the bowstring at brace. During the draw, the archer represents the driving component with respect to cam rotation. Upon release of the bowstring, the limbs become the driving component. During the brief, initial stages of launch, the limbs are driving the very small radius of one cam lobe that, by virtue of being integrally attached to the associated cam lobe, is essentially driving a lobe of considerably larger radius. At this interval, the speed of the bowstring travel is substantially greater than that of the speed of the cable, the speed of the cable being directly proportional to the speed of recovery of the limb. However, as the sequence of ratio relationships of one lobe to the other progresses through the launch, the effect of amplification of rate of travel of the bowstring as related to the rate of travel of the cable progressively diminishes and eventually reverses. In other words, the rate of bowstring travel is constantly slowing down throughout the launch as a seemingly unavoidable result of the design of the cam that is necessary to achieve the desired draw/force characteristics. Because this aspect of influence upon arrow velocity appears to be unavoidable, it has either been unrecognized or ignored altogether. Of course, if the relationship of the speed of travel of the bowstring to that of the speed of recovery of the limbs could be improved, this would improve arrow velocity. Such an improvement in arrow velocity would also demand a consideration of the fact that a substantially higher rate of arrow velocity may be attainable for a given range of stored energy beyond that exhibited with conventional compound bow design.
The present invention improves the speed of bowstring travel relative to the recovery speed of the limbs thereby overcoming the seemingly unavoidable condition inherent in conventional design as outlined above. This is accomplished through the implementation of the take-up spool/cam drive wheel component of the previous design for the purpose of providing an additional, intermediate ratio that offsets or compensates for the diminishing effect of the cam with respect to the rate of speed of bowstring travel. The take-up spool/cam drive wheel component of the previous design was intended solely as a means of reducing the overall size of a compound bow and in the patent(s) pertaining to the previous invention, this component was not recognized as a means to any other end. The present invention, however, proposes that, while the intended use of the component is incorporated for the original purpose of providing for an ultra-compact configuration, surprisingly new results are produced when that component is utilized with other elements of the invention.
The ability of various bow components (primarily the riser) to resist the forces imposed by the deflecting limbs, along with loaded cables and bowstring, substantially affects the performance of the bow in terms of both arrow velocity and accuracy. Components that are not uniformly loaded or designed specifically to compensate for non-uniform loads tend to deflect or bend under load. Any such deflection, particularly in the riser, adversely affects both arrow velocity and accuracy.
Riser deflection is a component of hysteresis. Hysteresis is generally defined as the difference between the work done in drawing the bow and the kinetic energy of the arrow as it leaves the bow. Because hysteresis is commonly an assessment of static friction, the primary focus of those seeking to reduce hysteresis has been at the bearings on which cams or idler pulleys rotate. Bearings made from improved materials or antifriction bearing such as roller or ball type bearings have been employed to reduce friction and have produced some reduction in hysteresis. However, the analysis of hysteresis in a compound bow cannot be limited to revolving or rotating components alone. Some portion of the static hysteresis of a compound bow is attributable to the components of the bow, such as the riser, deflecting under the imposed loads. In a compound bow, due to limitations necessary to provide clearance for the arrow, cables, and other elements, a degree of offset and non-balanced loading is perceived to be unavoidable in the design of compound bows. One example is that of the spacing of the tracks of the lobes of the cam that carry the cables and bowstring with respect to the center of the axle on which the cam rotates. During the drawing of the bowstring, loads in the bowstring change constantly and, simultaneously, loads in the cables change constantly as well, creating a constant transfer of loads from one side of the centerline of the axle to the other. This creates what is commonly referred to as cam lean, limb lean, or limb twist. Twisting limbs impose lateral loading on the riser at the limb pivot location of the riser as well as through the neutral axis of the riser via loads in the cables. This imbalance is aggravated by the need to offset that portion of the riser defining the arrow pass and sight window area. Further imbalance results from moving the cables that span from the upper to the lower limbs, crossing in the proximity of the arrow path, out of the way in order to provide clearance for the fletching of the arrow. This is commonly accomplished by a cable guard rod extending from the riser to the location of the cables and, by means of a sliding attachment, relocating the cables to a position clear of the arrow-fletching path. This relocation or offset of the cables creates an angular attitude of each cable with respect to the plane of the axle and thus a further load imbalance.
The adverse effect of non-balanced loading are not recognized as a component of hysteresis, although it is recognized as a factor with respect to accuracy. It is known that the deflection or bending that occurs in a riser—along with cam lean—affects the travel of the nock and thus the arrow path during launch. Any deviation from a perfectly straight path of the nock induces oscillation to the arrow and causes erratic arrow flight. Therefore, much attention has been devoted toward the design of risers, to resist bending under load to reduce cam-lean. As yet, however, no compound bow has been designed in which the results of unbalanced loading can be regarded as inconsequential.
As stated above, imbalanced loading of limb tips and subsequent limb twist is an inherent problem of prior art bows, whether of one or two cam design. On conventional one or two cam bows, this imbalance is a result of the necessity to orient the lobes of the cam(s) such that the bowstring and buss cables will be in their required positions laterally and the fact that the loads in the string and cables constantly change throughout the draw. This imbalance is further aggravated on conventional bows by the requirement to move the buss cables, which cross from top to bottom in the vicinity of the arrow pass, forcibly toward the string-hand side in order to clear the arrow fletching as the arrow passes the cables. In the bow example of my previous invention, adequate fletching clearance was achieved by means of locating the minor lobe of the cam off center and likewise setting the buss cable attachment point at the limb tip off center. However, this arrangement created a more pronounced imbalance at the limb tip and thus more pronounced limb twisting.
Yet another aspect related to the tendency of a bow riser to bend or deflect in an undesirable manner relates to the tendency of the upper and the lower portion of the riser to bend back toward the archer about the center or grip portion of the riser as a result of the force of the archer's grip in opposition to the forces of the limbs being deflected by the cables and bowstring. On conventional bows, the attitude of the mounting of the limbs is such that the limbs are more parallel to the vertical plane of the riser than parallel to one another with respect to their length. Thus the forces applied to the riser at the pivot point of each limb under deflection is directed back toward the archer and in opposition to the force applied to the grip portion of the riser by the archer. Additionally, forces in the buss cables directed from top to bottom and bottom to top of the bow add to the reaction of the riser about the pivot point at the grip.
Deflection or bending in the riser in this manner is likewise detrimental to both performance and accuracy and some attention has been given to the reduction of this effect. Risers have been designed with strut-like appendages that span or bridge from the top of the riser to the bottom in order to counteract these forces and reduce riser bending. The geometry of the bow of the present invention, however, provides a limb mounting configuration in which the limbs are more parallel to one another and more perpendicular to the vertical plane of the riser (this geometry was first disclosed in my previous patent). The geometry of the present bow also isolates limb and cable forces at either end of the riser, so the only forces now relevant to riser bending are those forces applied by the archer in drawing the bowstring. Since these forces alone represent far less that those which would be required to exceed the capability of the riser material to resist bending, it may be seen that the present bow all but eliminates undesirable riser bending.
All cables experience some permanent elongation when first loaded. Afterward, some additional, elastic stretch occurs each time an additional load is applied. The amount of initial elongation and subsequent elastic stretch is depends in part on the specific material used to construct the cable as well as its method of construction. Cable stretch causes undesirable effects in compound bows. The initial elongation that occurs allows limb tips and cam positioning to become displaced from their intended starting position at brace thereby altering the force/draw characteristics. One can compensate for initial, permanent elongation by pre-stretching the cable prior to installation. However, the elastic stretch that occurs thereafter remains and the amount of stretch is proportional to the length of the cable. Shorter cables absorb less energy than long ones, making more energy available to the arrow and thus improving arrow velocity. The buss cables of the bow of the present invention are approximately 80% shorter than that of conventional compound bows, and reduce the effect of elastic stretch to insignificant levels.
On a conventional two-cam type compound bow, the individual cams are directly linked by the bowstring and, with the buss cables from each cam resisting the force from the opposing limb; the cams tend to rotate together. However, owing to the geometry of the bow, the cams do not inherently work in perfect synchronization. Unless the bowstring is drawn from the precise center-point between the cams, which is not normally the case, the length of bowstring from the draw point to one cam will be different than that of the length of bowstring to the opposite cam. This causes one cam to rotate at a different rate from the other. A great deal of attention has been focused on cam synchronization within the art and a number of methods have been developed to improve cam synchronization in two-cam bows.
The most significant development aimed at eliminating problems related to cam synchronization was the one-cam bow, discussed previously. The simple fact that the one cam bow design has only one cam obviously eliminates cam synchronization as a problem. Nevertheless, the one-cam design presents a number of other problems related, in part, to cam timing, nock travel, bowstring and cable angles, unbalanced loading of the riser and other components, cam lean, and subsequent performance characteristics.
The limbs of a compound type bow typically are attached to the riser by a limb bolt which provides a way to adjust the limb pre-load tension in order to alter the peak draw weight of the bow. Conventional compound bows generally utilize one of two methods of anchoring the limb bolt.
The more common method is to pass the limb bolt through an opening in the limb located at the butt end of the limb and then to insert it directly into a threaded hole provided in the riser. While this arrangement is an economical way of providing adjustability, it has certain disadvantages. Because the bolt threads directly into the riser, the bolt is exactly perpendicular to the limb at only one point of adjustment. This point is usually where the limb is positioned such that the pre-load deflection of the limb provides the greatest available peak draw weight. From that point, the limb bolt (for each limb) may be unscrewed in order to reduce the pre-load and thereby provide a downward adjustment in peak draw weight. However, the amount of reduction is limited because, as the bolt is unscrewed the limb rotates about its pivot point resulting in an angular relationship between the fixed path of the bolt and the plane of the limb. While countersunk head type bolts are commonly used in conjunction with truncated washers to provide for some misalignment, the amount of available travel of the limb bolt is minimal before binding occurs either at the bolt head or between the body of the bolt and the side of the opening in the limb through which it passes. Although this arrangement provides adequate thread engagement to resist the forces imposed by the limb on the bolt and riser connection, it provides a limited range of adjustment making it necessary to use a bow press type device to assemble the bow and preload the limbs or to perform maintenance operations such as replacing a bowstring or cables. Furthermore, this arrangement is subject to the possibility of severe damage to the limbs or other components if the range of adjustment is exceeded.
The other method is designed to minimize the undesirable consequences that may arise from binding caused by an angular relationship between the limb bolt and the limb. In this method, the limb bolt passes through an opening in the butt end of the limb and uses a countersunk head bolt and truncated washer as above. However, instead of providing a threaded hole directly into the riser, a pocket type opening is provided in the riser through which the bolt may reach a cylindrical bar mounted laterally through openings in the sides of the riser and providing a threaded hole through the cylindrical bar to engage the threads of the bolt. This arrangement allows the bolt to pivot about the axis of the cylindrical bar such that a perpendicular attitude with respect to the limb may be maintained within the range of adjustment thereby avoiding binding. However, because a certain amount of material of the riser must be available above the openings for the cylindrical bar to resist the forces transmitted by the bolt, the cylindrical bar is restricted in terms of diameter thus providing a greatly reduced range of thread engagement and a restricted amount of adjustment in the limb preload. In this arrangement, the use of a bow press is also necessary, as no additional adjustment is made available. In fact, due to the limited range of thread engagement in this method, either extreme caution or some mechanical stopping means is required when reducing limb tension by unscrewing the limb bolts in order to avoid a bolt becoming disengaged from its anchor, resulting in an uncontrolled release of limb tension. The present invention provides a limb bolt adjustment means that eliminates the shortcomings of the previously described bows.
Nock travel is defined as the path that the arrow nock point on the bowstring takes as the bowstring is drawn by the archer, and subsequently the path that the arrow takes upon launch. Ideally, nock travel is virtually level and perpendicular to the riser when viewed from the side of the bow and defining a straight path, free of side-to-side movement, when viewed from the front of the bow. In prior bows, ideal nock travel is difficult to achieve for a number of reasons, one being that in most art bows, the nock point on the bowstring is not located at the center point between the limbs. Thus a greater length of bowstring below the arrow is affected by the draw than the bowstring above the arrow, resulting in an unlevel nock path when viewed from the side of the bow. Yet another reason for less than ideal nock travel is non-uniformity of components such as the cam and idler: this affects bowstring travel. In many cases, innovative cam designs, particularly in one-cam bows, have successfully achieved a relatively level nock path in relation to the bow as viewed from the side. However, the aspect of lateral nock travel, as viewed from the front of the bow and in relation to the vertical plane of the riser, has not been successfully addressed. Previously, it was mentioned that limb twist and riser deflection adversely affect both accuracy and performance. This can be directly related to lateral nock travel. In most conventional prior art bow designs, inherent limb twist, cam or idler lean, and lateral deflection of the riser caused by unbalanced loading results in the bowstring deviating from a straight lateral path. Upon launch of the arrow, this deviation manifests itself in the form of arrow oscillation affecting both performance and accuracy in an undesirable manner.
Considering the factors relating to compound bow performance, both recognized and unrecognized, it is evident that a bow which resolved factors related to unbalanced loading would result in a substantially improved arrow velocity, accuracy, and overall performance.
On any two-cam bow, cam synchronization is important. With conventional two-cam bow designs, where the arrow nock point is not located at the center of the bowstring between the limbs, cam synchronization is a problem in that the cams must be uniquely designed or provided with some form of adjustment so as to compensate for the unequal lengths of bowstring above and below the arrow nock point so that both cams can rotate in the appropriate relationship to one another and arrive at the proper ending (full draw) position at the appropriate time. Improper cam synchronization will result in a loss of both accuracy and efficiency.